feat: use captain endpoint config in legacy OpenAI base service (#12060)

This PR migrates the legacy OpenAI integration (where users provide
their own API keys) from using hardcoded `https://api.openai.com`
endpoints to use the configurable `CAPTAIN_OPEN_AI_ENDPOINT` from the
captain configuration. This ensures consistency across all OpenAI
integrations in the platform.

## Changes

- Updated `lib/integrations/openai_base_service.rb` to use captain
endpoint config
- Updated `enterprise/app/models/enterprise/concerns/article.rb` to use
captain endpoint config
- Removed unused `enterprise/lib/chat_gpt.rb` class
- Added tests for endpoint configuration behavior

spec/lib/integrations/openai/processor_service_spec.rb

@@ -253,5 +253,52 @@ RSpec.describe Integrations::Openai::ProcessorService do
         expect(result).to eq({ :message => 'This is a reply from openai.' })
       end
     end
+
+    context 'when testing endpoint configuration' do
+      let(:event) { { 'name' => 'rephrase', 'data' => { 'content' => 'test message' } } }
+
+      context 'when CAPTAIN_OPEN_AI_ENDPOINT is not configured' do
+        it 'uses default OpenAI endpoint' do
+          InstallationConfig.find_by(name: 'CAPTAIN_OPEN_AI_ENDPOINT')&.destroy
+
+          stub_request(:post, 'https://api.openai.com/v1/chat/completions')
+            .with(body: anything, headers: expected_headers)
+            .to_return(status: 200, body: openai_response, headers: {})
+
+          result = subject.perform
+          expect(result).to eq({ :message => 'This is a reply from openai.' })
+        end
+      end
+
+      context 'when CAPTAIN_OPEN_AI_ENDPOINT is configured' do
+        before do
+          create(:installation_config, name: 'CAPTAIN_OPEN_AI_ENDPOINT', value: 'https://custom.azure.com/')
+        end
+
+        it 'uses custom endpoint' do
+          stub_request(:post, 'https://custom.azure.com/v1/chat/completions')
+            .with(body: anything, headers: expected_headers)
+            .to_return(status: 200, body: openai_response, headers: {})
+
+          result = subject.perform
+          expect(result).to eq({ :message => 'This is a reply from openai.' })
+        end
+      end
+
+      context 'when CAPTAIN_OPEN_AI_ENDPOINT has trailing slash' do
+        before do
+          create(:installation_config, name: 'CAPTAIN_OPEN_AI_ENDPOINT', value: 'https://custom.azure.com/')
+        end
+
+        it 'properly handles trailing slash' do
+          stub_request(:post, 'https://custom.azure.com/v1/chat/completions')
+            .with(body: anything, headers: expected_headers)
+            .to_return(status: 200, body: openai_response, headers: {})
+
+          result = subject.perform
+          expect(result).to eq({ :message => 'This is a reply from openai.' })
+        end
+      end
+    end
   end
 end